1. Field of the Invention
The present invention relates to a biological information measuring apparatus for acquiring various types of biological information associated with the metabolism of a living organism, such as biological density, water content, blood oxygen level, oxygen saturation, glucose level, blood glucose level, and pulse, on the basis of a phenomenon that propagation of light through the living organism changes differently depending on the wavelength of the propagating light.
2. Background Art
In recent years, there have been actively proposed, as an apparatus and method for conveniently analyzing the interior of a living organism in a noninvasive manner, apparatuses and methods for acquiring information on the interior of a living organism by emitting light from a light source provided on the surface of the living organism to the interior of the organism, and receiving reflected light which has propagated through the living organism while being scattered and absorbed therein, and which has reached the living organism surface. For example, in Japanese Patent No. 3623743, the applicant of the present application has proposed a biological information measuring apparatus which measures biological information by making use of spread-spectrum modulation and demodulation. This biological information measuring apparatus can acquire information, for example, at the cerebral cortical layer through measurement of oxygen level in blood on the basis of a change in absorption of near-infrared light by hemoglobin.
In the apparatus disclosed in Japanese Patent No. 3623743, a light emitting section is configured to spread-spectrum modulate a signal to be transmitted (a base band signal) and emit light from a semiconductor laser. As is known, the power spectrum of a spread-spectrum modulated signal is generally represented as shown in FIG. 12. Thus, light output on the basis of a signal having a power spectrum as shown in FIG. 12 has a large output (intensity) at direct current (DC) and frequencies in the vicinity of DC. When information of a living organism is measured by use of light having such a power spectrum, the measurement is likely to be affected by DC component variations (e.g., drift variation, offset variation, and 1/f noise) of a light source driver, a light source, an optical detector, and an amplifier; i.e., so-called DC problems associated with spectrum spreading, so that, although relative measurement is possible, strict measurement may be difficult.
Further, in the apparatus disclosed in Japanese Patent No. 3623743, a light detection section is configured to convert light having propagated through a living organism to an electrical detection signal (analog signal), and convert the detection signal (analog signal) to a digital signal by means of an AD converter. However, this patent does not clearly show a sampling frequency at which the AD converter converts an analog signal to a digital signal. For example, in the case where the conversion processing is performed at a sampling frequency corresponding to the emission period of light, a detection signal (analog signal) corresponding to received light cannot be sampled over the entire signal band (frequencies) thereof, so that the detection signal (analog signal) may fail to be properly reproduced in the form of a digital signal. That is, the conventionally practiced measurement of the interior of a living organism by making use of spectrum spreading cannot be said to effectively use frequencies necessary for emission of light, detection of light, conversion of detected light to an electrical signal, etc. Accordingly, the conventional apparatus may fail to properly and reliably obtain information regarding a living organism, which information is carried by light having propagated through the living organism.